The present invention relates to systems for initiating and enhancing combustion of fuel and fuel-air mixtures and deals more particularly with a system for increasing the efficiency with which the electrical discharge energy is coupled into the fuel by ignition enhancement devices.
Initiation of fuel combustion for compression-type internal combustion engines is a well developed art which has its origin in the Otto-cycle Spark Ignition engine that was developed in the late 1800's.
The first ignition systems employed a high voltage magneto that provided the electrical energy to the spark plug according to the position of the engine.
The magneto was gradually replaced during the 1920's by a battery-based induction coil system (Coil Ignition system - C.I. or Kettering system). In these systems, before ignition, the low voltage electrical energy (typically 12 volts) is first transferred from the battery into the primary winding of the coil through mechanical breaker points and generates a high electro-magnetic field in the coil. At ignition point, a cam opens the breakers, modifying the field and generating a voltage (typically 20,000 volts) in the secondary high voltage winding of the coil which is applied to the spark plug such that the spark plug gap breaks over and transfers the energy to the air-fuel mixture.
In the case of typical multi-cylinder engines, a high voltage distributor, made of a rotor and a distributor cap, directs the energy to the appropriate spark plug according to the engine crankshaft position through auxiliary air gaps.
The advent of reliable semiconductor device, some 30 years ago, introduced technology which led to the gradual elimination of performance limitation and maintenance problems associated with the mechanical breaker. Transistor-assisted-contact systems (T.A.C.) were introduced where a transistor device relieves the mechanical breaker points of the burden of carrying high current.
More recently, mechanical breaker points have been entirely replaced by opto-electronic or inductive sensors coupled to electronic timing and driver circuitry that directly control the coil primary winding current (Transistor Coil Ignition system - T.C.I.).
Recently efforts have also been made to eliminate the conventional mechanical rotor system for high voltage ignition pulse distribution, mainly in using multiple coils (one coil per spark plug) or coils with multiple windings associated with high voltage diodes (several spark plugs connected to the same secondary coil winding, plug selection made by using energy polarization).
The availability of high power fast switching devices (Metal Oxide Semiconductor Field Effect Transistors--M.O.S.F.E.T., thyristors, for instance) has given rise during the last decades to a variety of capacitor Discharge Ignition systems (C.D.I.).
In these later systems, in contrast to the Kettering system, the energy is stored from the battery into a medium voltage (about 400 volts) capacitor before ignition (using an inverter that converts the 12 volt battery voltage to the desired level); then, at ignition point, the energy is transferred to the spark plug through a high voltage semiconductor switch and a step-up transformer which provides the 400 to 20,000 volt conversion.
Modern conventional coil ignition systems and capacitor discharge systems (C.I., T.C.A., and C.D.I.) usually deliver between 5 and 100 milliJoules (mJ) of electrical energy per spark pulse at a peak output voltage ranging from 20,000 to 30,000 volts. The more common systems operate in the energy range of 20 to 50 mJ per pulse.
In C.I. and T.C.A. ignition systems, the output voltage (across the spark plug gap) rise time ranges from 60 to 200 microseconds (.mu.S), due to the electrical characteristics of the ignition coils. The spark duration mainly depends on the physical size of the coil, but typically ranges between 1 and 2 milliseconds (mS).
In contrast to the inherently slower longer lasting output pulse characteristics of the coil ignition systems, C.D.I. systems provide faster rising pulses (typically 1 to 50 .mu.S) at the expense of shorter overall duration for a similar output pulse energy.
The faster rising pulses of the C.D.I. systems are less susceptible to misfire due to spark plug fouling (gap breakdown voltage not reached as all energy dissipated during the rise of the pulse in the plug insulator deposits).
The faster rising pulses of the C.D.I. systems are less susceptible to misfire due to spark plug fouling (gap breakdown voltage not reached as all energy dissipated during the rise of the pulse in the plug insulator deposits).
The overall duration of a C.D.I. ignition pulse could be increased for better ignitibility in most operating conditions; however, that would be made at the expense higher output pulse energy and reduced spark plug lifetime.
Gaseous electrical discharge typically occurs in three phases as follows:
1) A breakdown phase, usually less than a few tens of nanoseconds, in which current flow increases rapidly as the voltage across the discharge gap falls.
2) A transition to arc discharge of relatively high internal energy content and current density.
3) A glow discharge characterized by lower internal energy and current density.
The overall duration of an ignition system discharge and the relative fraction of total energy dissipated during the breakdown, arc and glow phases are primarily governed by the circuit parameters of the system.
The discharge circuits of conventional coil ignition and transistor coil ignition systems typically have high inductance, low capacitance and relatively high resistance. These high impedance systems couple only a small fraction of the discharge energy into the fuel mixture during the breakdown phase and have the feature of relatively quick transition from breakdown to a long duration low current glow discharge.
Capacitive discharge ignition systems generally deliver a current pulse consisting primarily of the arc phase, due to their low output circuit impedance characteristics.
Recently the establishment of strict exhaust emission standards and a demand for better fuel efficiency have placed additional constraints on engine operation. In response to these demands, recent trends in engine design and operation have been toward promoting a better combustion process and extending stable operation to leaner fuel mixtures.
It has been experimentally established that minimum spark ignition energy requirements correspond to fuel mixtures which are at the stoichiometric ratio. This mixture range corresponds to an air-to-fuel mass ratio of about 14.7:1 (or excess air factor lambda=1). This mixture provides maximum laminar flame velocity and maximum engine power output, and it is the mixture with which engines operated prior to the 1970's.
While engines show proper stable operation and driveability when operating at excess air factor ranging 0.85 to 1.15 with conventional ignition systems and engine design, emissions vary greatly within this range. As shown in FIG. 1, emissions of hydrocarbons (HC) and carbon monoxide (CO) decrease with increasing excess air factor in the range mentioned above but emissions of oxides of nitrogen (NOx) increase.
Due to recent legislation on emission control in several countries including USA, Japan, Switzerland, Austria, Sweden, and Canada which has placed limits on the 3 above exhaust gas constituents (HC,CO and NOx), it has been necessary to reduce emissions for instance by exhaust gas after-treatment (thermal after-burning or catalytic after-burning) as emission levels are exceeded at any air factor within the above mentioned engine operating range. Such legislation is likely to be introduced in most countries and to become more and more stringent.
Most common current exhaust gas after-treatment uses three-way or selective catalyst with excess air factor sensor. Operation efficiency of such after-treatment is shown at FIG. 2. The system works such that at stoichiometric ratio the conversion efficiency of the catalyst is satisfactory for the three emission constituents.
Operation with exhaust gas recirculation (E.G.R.) diluted mixtures can achieve significant reductions in exhaust nitrogen oxides emissions. Increasing E.G.R. tends to lower peak combustion temperature which in turn reduces NOx generation.
Conversely, operation with a diluted mixture is characterized by more difficult ignition and slower laminar flame velocity which eventually leads to cycle-by-cycle (C.B.C.) variations, incomplete combustion and a subsequent increase in unburned hydrocarbon emission.
Promoting better combustion initiation and enhancement reduces C.B.C. variations and permits more dilute fuel mixtures up to a level where NOx could be reduced to a level well below regulation limits and exhaust gas after-treatment could concentrate on CO and HC constituents only and with higher efficiency.
Known ignition enhancement systems usually operate at higher energy levels, ranging from about 60 mJ to several joules per pulse.
Some systems provide a single long lasting glow discharge which yields effective ignition kernel durations from 2 to 10 milliseconds. These systems may use either a larger ignition coil, resulting in undesirable spark plug electrode erosion, or two ignition coils alternately triggered to maintain the discharge, resulting in a highly complex system arrangement. Both systems also suffer from poor combustion initiation performance (short arc discharge) when operating with air-fuel mixture ratio in the region of 20:1 or E.G.R. diluted mixtures.
Other systems use a series of several short discharges generated from C.D.I. systems. Again they yield high system complexity which renders them impractical for commercial use in engines.
Another system covers Plasma Jet Ignition (P.J.I.). This system has undergone considerable investigation during the 1970's and has been shown to be very effective in promoting leaner engine combustion. However, this system is undesirable from the standpoint of electrode erosion.
Another system covers Hard Discharge Ignition (H.D.I.). This system shows high complexity and has not yet proved able to run with highly diluted mixtures.